LiFePO4 - The future for off-grid battery banks?

[Jonathan Cole]

Hi Steve,

LiFePo cells have a higher nominal voltage than lead acid batteries and usually the charge controller is programmed for specific batteries and their optimal charging characteristics. No charge controllers that I know of are set up for these batteries. What make of solar charge controller did you use?

By the way, an MPPT charge controller only makes economic sense if you live in the high cool lattitudes. Otherwise it is more cost effective to purchase a pulse width modulation charge controller which is less than half the price and has more than twice the warranty (5 years vs. 2 years). You are better off getting the cheaper charge controller and spending the savings on additionall PV panels which have a 20-25 year warranty.

Jonathan

[john p]

Steve G mostly my fault as I had made the post look like it was a question from me have redone the posts so no more confusion..

You find anything useful in my posts?

[steveg]

Sorry - my bad.

[john p]

Steve g that was my reply to Josko you can read my other 3 posts in this thread.. It will explain my experiences with LFP batteries..

[steveg]

Hi John,

As in the other thread - your BMS (if any) is relative to your end application.

My LiFePO4 pack for my home has a simple BMS built into it - it protects the cells from over and under voltage with weak balancing. Other BMS'es have strong balancing as they need to miximise the power form the cells, but the BMS is more expensive.

You *will* need a solar regulator for charging the batteries from Solar PV cells - MPPT gives you more efficiency. If you want the most plain simple solution, you just need to regulate to the charge voltage/cell (3.65V in most cases for LFP)

But as I said depends on your application.

Over cell voltage will definately damage the cell ... consider this ...

you have a 200W panel the no load volatge is 45V, but if you put a 24V cell on the cell without a regulator in full sun the voltage will sag to 'charge' the cells, but the panel will continue to want to drive the cell all the way to 45V ultimately... another possible option is to size the battery pack to suit the cells, but you will get baqd efficiency of charge.

So for a 45V output cel you will put 13 LFP cells in series (to avoid over charge), but the cells will only charge very slowly - so a regulator is the best way - an MPPT one giving the best efficiency.

[john p]

It looks that you have some experiences with LFP batteries. Could you advise me if there is still a need to install solar charge regulator if you use BMS for LFP batteries? Josko A BMS only regulates individual cells .. You certainly still need a charge controller to safely charge the batterypack

[josko]

Addendum : you may notice I am one panel shy on the bottom row - due to a mis calculation one of the bathroom vents is where my panel should be - we will have this fixed in due course, so I have 3500W solar hooked up right of this minute - the last 2 panels once the vent is moved.

The top row is 200W panels of Suntech the lower row is 250W 'no name' brand with Suntech wafers that I imported myself from China - the battery pack came from the same folks, to my spec (3x100Ah in parallel) x 16 in series

The BMS prevents overcharge and overdischarge - there is some weak balancing in the pack to bring the voltages over time to even values.

Been running now for 5 months.

Also - we have insurance and have passed code of compliance for our LED lighting internally (new technology that councils dont trust especially running under insulation) and the whole off-grid thing ... as the batteries are inherently safe and there is plenty of test data for LiFePO4 for impact penetration short circuit etc. as a *technology*

The wiring was done by registered electrician (I chose one that also did solar PV installs) and was signed off by same as this is required to get code of compliance which is needed before you can insure your home in NZ.

2 days after code of complaince we had a 7.1 earthquake and nothing was damaged, so light battery packs need less mounting and securing too btw... for those elsewhere this is the same size as the Haiti quake...

It looks that you have some experiences with LFP batteries. Could you advise me if there is still a need to install solar charge regulator if you use BMS for LFP batteries?

[steveg]

The total cost was approx 10% of the cost of the house to build (maybe a little less) - it has no payback because it is part of the house. If you purchase a car you get the utility of the car, it has no 'payback', and when you sell that car you get less for it. So for the system I have, I view it in this way also - we pay an extra for it on our house, we get the utility of it for (however long) but it doesnt devalue quite as quickly. I get to have no electricity or gas bills, and once my trees I cut are dry no costs for wood either.

You say it costs a lot to live in New Zealand? - it depends on your income - I lived in Sydney for 10 years, and it definately costs more to live there in raw $ , so it's all relative - average incomes are less in NZ is probably a more accurate statement.

[steveg]

Hi Leon,

US$7000 for the battery pack yes - this is equivalent to >1000Ah of PbA, but lighter smaller.

I paid NZ$2000 for solar electrics install - so that is wiring the battery pack in and up to the box in the roof to take the solar panel feeds - wiring the regulator in, fusing and approval.

Lets see - right now 1NZ$=about 0.75USD - say US$1500.00

and the panels cost US$2.1/watt at the time

[Sunking]

SteveG, you said that your pack was $7000 U.S., are you saying the complete package

That would have to be the batteries alone. 4000 watt of solar panels will cost $8000 or more.

[russ]

Read about your earthquake on the net! Amazingly little damage.

Strong building codes do pay off.

[leonscorpio]

SteveG, you said that your pack was $7000 U.S., are you saying the complete package including installation? I am only asking since I have family that live in NZ and they tell me it's quite expensive just to live.
I'm still learning on the whole solar thing, am looking at getting one installed in Georgia soon.
take care

[steveg]

also ...

Addendum : you may notice I am one panel shy on the bottom row - due to a mis calculation one of the bathroom vents is where my panel should be - we will have this fixed in due course, so I have 3500W solar hooked up right of this minute - the last 2 panels once the vent is moved.

The top row is 200W panels of Suntech the lower row is 250W 'no name' brand with Suntech wafers that I imported myself from China - the battery pack came from the same folks, to my spec (3x100Ah in parallel) x 16 in series

The BMS prevents overcharge and overdischarge - there is some weak balancing in the pack to bring the voltages over time to even values.

Been running now for 5 months.

Also - we have insurance and have passed code of compliance for our LED lighting internally (new technology that councils dont trust especially running under insulation) and the whole off-grid thing ... as the batteries are inherently safe and there is plenty of test data for LiFePO4 for impact penetration short circuit etc. as a *technology*

The wiring was done by registered electrician (I chose one that also did solar PV installs) and was signed off by same as this is required to get code of compliance which is needed before you can insure your home in NZ.

2 days after code of complaince we had a 7.1 earthquake and nothing was damaged, so light battery packs need less mounting and securing too btw... for those elsewhere this is the same size as the Haiti quake...

[steveg]

My Off-Grid Home

Hi Russ,

We have just moved into our new off-grid home - we went off-grid because we wanted to - there is power to our street and is only 130metres away from the house.

We have solar hot water and wetback fireplace - we use the fireplsace to cook in winter when there is not enough power to use the oven and stove top (induction cooking)

We are a family of 5 my kids are 16,13,10 so plenty of internet use and TV - we have a 42" LCD tv we use in the evenings and weekend mornings for cartoons.

The house is 320m^2 including a triple garage - due to regulations we spent a large amount on insulation and double glazing.

In winter we have a power footprint of 5.5 to 6kWh using the fireplace to cook and in summer 12-15kWh per day, when we also use the dishwasher and water the garden (water pump)

I have 4kW of solar panels and have the house designed to face North (southern hemisphere - New Zealand)

I have a 7kW inverter which runs all our stuff - and the battery pack.

As soon as I figure out how I'll post pictures of the house and system.

it is early/mid spring here, so on a sunny day we get a full charge from the night before by about 11am (6-7kWh) and the extra while the sun is out my wife burns power in any way she wants to... (hence the 12-15kWh)

The charge efficiency of LiFePO4 is very high and we live in the top 30-40% of the battery so I expect my lifetime of the cells to be high.

[russ]

@ steveg - Welcome to Solar Panel Talk!

We would love to hear more about your system!

Russ